Genetic variation and evolution

Genetic Variation and Evolution

• Genetic variation refers to differences in gene alleles within a population
• Natural populations contain significant genetic variation
• Evolution is how an entity changes over time, developed by Darwin as "descent with modification"
• Species accumulate differences over time, causing descendants to differ from ancestors and new species to arise from existing ones
• Natural selection is a mechanism of evolution that can change allele frequencies

Population Genetics

• Studies gene properties within a population
• Evolution brings about a change in how the genes are made up
• Natural populations contain a lot of genetic variation
• Genetic variation is needed for evolution

Genetic Variation

• It is now measured using advanced tools
• Human blood groups serve as an example of variation caused by genetic differences
• Single Nucleotide Polymorphisms (SNPs) are used to assess patterns in over 300 species
• The 1000 Genomes project is designed to sequence genomes from 2,500 people across five major global areas
• Genome regions vary, some showing more variability than others

Genetic Variation (cont.) and Blending Inheritance

• Genetic variation puzzled Darwin and his contemporaries
• Mendel's work was largely unknown
• Scientists once believed selection favored an optimal form, eliminating variation
• Blending inheritance, the widely accepted concept at the time, suggested offspring would have traits intermediate to their parents

Hardy-Weinberg Principle #1

• This principle predicts genotype frequencies
• Hardy-Weinberg equilibrium specifies that genotype proportions remain constant if:
  • No mutation occurs
  • No gene transfer occurs to or from external sources
  • Random mating takes place
  • The population size is very large
  • No selection takes place

Hardy-Weinberg Principle #2

• The principle can be written as an equation to work out allele frequencies
• If "p" is the frequency of the first allele and "q" is the frequency of the second allele, p + q = 1
• The equation is as follows: p2 + 2pq + q2 = 1 or BB + Bb + bb = 1

Making Hardy-Weinberg Predictions

• Allele and genotype frequencies remain constant if 5 assumptions for Hardy-Weinberg equilibrium hold true from one generation to the next
• Natural populations rarely meet all five assumptions
• The primary use of the equation determines if evolutionary processes are operating in a population and to suggest what they are

Hardy-Weinberg Equilibrium Variations

• Natural selection can favor homozygotes over heterozygotes
• Individuals may choose mates with similar genetics
• The influx of individuals can join in from other populations
• Mutations can occur

Agents of Evolutionary Change

• There are five agents of evolutionary change
• Mutation rates are generally low, and allele frequency is more commonly altered by other evolutionary processes making this an ultimate source of genetic variation
• Gene flow moves the alleles between populations, such as when an animal migrates or gametes drift into a different area
• Nonrandom mating includes assortative mating between phenotypically similar individuals, increasing homozygous individuals, and disassortative mating between different individuals, producing more heterozygotes
• Genetic drift results in allele frequency changing by chance alone in small populations
• Selection includes both artificial where a breeder selects desired characteristics, and natural where the environment determines which individuals produce the most offspring

Agents of Evolutionary Change (cont.)

• Assortative mating is nonrandom and involves phenotypically similar individuals mating, increasing homozygous individuals
• Dissassortative mating involves phenotypically different individuals mating, causing excess heterozygotes

Genetic Drift

• Allele frequency can change by chance alone in small populations
• A population must be large to maintain H-W equilibrium
• Genetic drift's magnitude is inversely related to the population size
• Genetic drift can cause alleles to be lost in isolated populations where uncommon alleles are more at risk
• Founder and bottleneck effects can both contribute to genetic drift

Founder Effect

• When one or a few individuals start a new, isolated population
• Some alleles are lost, while others change in frequency
• This is common with organisms on islands, self-pollinating plants, and in Amish populations

Bottleneck Effect

• A drastic reduction in population size due to drought, disease, or other natural forces
• Survivors may only represent a random genetic sample of the original population
• This results in a loss of genetic variability

Selection

• Some individuals produce more progeny than others, which is affected by phenotype and behavior
• Artificial selection occurs when a breeder selects desired characteristics
• Natural selection occurs when environmental conditions determine which individuals produce the most offspring

Natural Selection Requirements

• Variation must exist among individuals in a population
• Variation must result in differences in the number of offspring surviving into the next generation
• Variation must have a genetic basis

Natural Selection Effects

• Natural selection is a process that can, but won't always, result in evolution
• Evolution is the historical record, or outcome, of change over time
• Evolution driven by natural selection causes populations to be better suited to their environment

Selection and Coloration

• Pocket mice come in different colors
• Darker coloration is favored by black lava rock, while lighter coloration is favored by sandy environments

Selection and Climate

• Enzyme allele frequencies often vary with latitude
• Enzyme lactate dehydrogenase allele frequencies in fish vary geographically
• Alleles formed by these alleles function differently at different temperatures

Selection for Resistance

• Widespread insecticide use has led to the rapid evolution of resistance in the over 500 known pest species
• Houseflies possess pesticide resistance alleles at the pen gene, decreasing insecticide uptake and the kdr and dld-r genes, decreasing target sites
• Disease-causing pathogens have evolved resistance to antibiotics
• With each new drug, resistant organisms emerge endangering human health

Fitness

• Fitness occurs when individuals with one phenotype leave more surviving offspring than individuals with an alternative phenotype
• The concept is relative; the most fit phenotype is simply the one that produces the highest number of offspring on average

Fitness Measurement

• The most fit phenotype is assigned a fitness value of 1
• In a toad population of green and brown toads, Green toads having an offspring number of 4, and brown toads having 2.5, the Green fitness is 4/4=1 and the Brown fitness is 2.5/4=0.625

Fitness Factors

• Fitness is composed of:
  • Survival
  • Sexual selection (success at attracting mates)
  • Offspring per mating
• Traits favored for one factor may disadvantage others
• Selection favors those with the greatest overall fitness
• Phenotypes with greater fitness typically increase in frequency

Reproductive Strategies and Sexual Selection

• Males and females typically differ in how they maximize fitness
• Females evaluate a male quality to decide whether to mate
• Female peahens prefer to mate with peacocks that have more eyespots on their tailfeathers

Parental Investment

• Parental investment refers to the energy and time each sex uses in producing and rearing offspring
• Females typically have a higher parental investment
• Male fitness increases by mating with as many females as possible because females are more restricted by the number of eggs produced, making females particular when picking a male that provide the greatest benefit
• Cases with biparental care can have a degree of mate choice that is equal
• Mormon crickets transfer a protein-containing packet to females during mating, making males the choosy sex
• Males that choose larger females leave more offspring

Sexual Selection

• Sexual selection describes competition for mates
• Intrasexual selection is associated with competitive interactions between members of the same sex
• Intersexual selection describes mate choice
• Secondary sexual characteristics are used to combat other males or persuade members of the opposite sex
• Sexual dimorphism describes differences between sexes with males larger than females
• Sperm competition contributes to features that increase the probability of a male's sperm fertilizing the eggs

Intrasexual Selection

• Individuals of one sex, usually males, compete among each other for the opportunity to mate
• Successful males might engage in an inordinate number of mating's while most others don't mate at all
• Elephant seal males controlling territories on breeding beaches is an example, with a few dominant males doing most of the breeding

Intersexual Selection

• It describes active choice of a mate
• Direct benefits of mate choice include selecting males who can provide better care
• Males may also maintain territories that provide food and refuge
• Indirect benefits of mate choice can result in higher-quality offspring based on health and genetics
• Handicap hypothesis states that only genetically superior mates survive with a handicap, still being debated

Sensory Exploitation

• Courtship displays appear to have evolved from a predisposition in females to respond to certain stimuli
• Sensory exploitation describes evolution in males of a signal taking advantage of preexisting biases
• Túngara frog males have a short burst of sound at the end of calls that females of this and related species are attracted to

Sensory Exploitation

• The opportunity for sensory exploitation may be widespread to see if particular stimuli in finches had latent preferences
• Birds with differently colored glued feathers resulted in females being strongly attracted to males with white crests
• mutations cause a male to have white crests that give them a great advantage

Maintenance of Variation

• The frequency a phenotype depends on the frequency within its population in frequency-dependent selection
• Rare phenotypes are favored in negative frequency-dependent selection as they are preyed upon less frequently
• Common forms are favored in positive frequency-dependent selection and tends to eliminate variation where “oddballs” stand out
• Oscillating selection favors phenotype at a time that maintains genetic variation in the population

Oscillating Selection and Heterozygote Advantage

• Medium ground finches of the Galápagos Islands illustrate oscillating selection with Birds with big bills favored during drought and smaller bills favored in wet conditions
• Heterozygote advantage favors heterozygotes over homozygotes to maintain both alleles in the population
• Sickle cell anemia is a hereditary disease impacting hemoglobin and causing severe anemia where homozygotes usually die prior to reproducing

Multiple Gene Selection

• Many traits can be affected by more than one gene and operates on the genes for the trait
• Different genotypes are favored, which will change the population
• Disruptive, Directional and Stabilizing selection are different types of selection

Disruptive Selection

• It is the act of eliminating intermediate types
• African black-bellied seedcracker finches have different beak sizes based on available seeds in two categories
• This favors beak sizes for one seed type or the other
• Birds with intermediate sized beaks are at a disadvantage with both seed types, being unable to open large seeds or efficiently process small seeds.

Directional Selection

• It eliminates one extreme, often when the environment changes
• Eliminating flies which moved towards the light
• Resulting population now have that behavior.

Stabilizing Selection

• Acts to eliminate both extremes by making intermediate more common
• Infants with intermediate weight at birth tend to have the highest survival rate

Experimental Studies

• Biologists have traditionally investigated what has happened in the past
• Fossils or DNA evidence show this
• Now there are recently started evolutionary lab and field experiments
• Laboratory studies include fruit flies that have been common for nearly 100 years

Guppy Natural Selection

• Guppy coloration differs in small South American streams
• Above waterfalls with other unable species, the barriers create two different environments due to different dispersal methods that leave predators rare
• Pike cichlid predators below waterfalls cause guppy males to be more drab and individuals to reproduce earlier

Guppy Studies

• Lab studies can be used to provide explanations that can test hypotheses about evolution in nature
• 10 large pools with added pike cichlids and killifish to make up four pools each and two to be used as controls
• Experiments confirmed genetic differences after about 14 months with 10 guppy generations
• killifish (and control) pool guppies turned out large and colorful and the pike cichlid pool contained smaller drab guppies

Interactions Among Evolutionary Forces

• Mutations and genetic drift may counter selection
• In nature, mutation rates are rarely high enough to counter selection, however selection is nonrandom and genetic drift is random
• Drift may decrease an allele favored by selection meaning selection usually overwhelms drift except in small populations.

Forces of Evolution

• Gene flow can be constructive, with beneficial mutations spreading to various populations or constraining, by continual flow of inferior alleles from other populations

Metal Resistance

• High metal ion concentrations are found in slim, abandoned copper mine bent grass
• Resistance confers slows growth.
• Resistance frequency 100% at mines, 0% elsewhere
• Gene flow can result in intermediate and higher copper levels.

Limits of Selection

• Similar genes affect the hen's comb size and rate at which she will lay making selection difficult because the is lack of genetic variation in thoroughbred horses
• Phenotypic variation may not have genetic basis which cause interactions between genes or cause an allele may vary from genotype.